Anti-farm code is the part of Guild Wars software used to reduce the benefits of repeatedly farming a single location, quest, or other objective. ANet has published few details about how it works; what little players know is based on limited experimentation and speculation.
 Official facts
Since the first implementation:
- Farming alone will yield fewer total items than farming with a full party, though more items per individual character.
- Certain items are not affected:
- Repeated map entry does not reduce loot (source).
 Player tests
These theories have been confirmed by multiple players performing a variety of tests under a variety of conditions.
- The drop rate from the first foes killed in the area is less than normal; after several kills, the rate returns to normal.
These hypotheses have strong advocates, but others point to contradictory evidence or insufficient data.
- Killing too many foes too quickly results in lost drops.
- Advocates of this theory recommend spike over AoE damage, to limit the number of simultaneous or near-simultaneous deaths.
- Critics point out that not every foe drops an item and that, necessarily, some of those missing drops will occur during simultaneous kills.
- The drops are seeded when the party enters an area.
- This was tested by having multiple parties enter the same zone at the same time. Researchers counted drops and also looked at drops from specific foes. Their results flew counter to the previous hypothesis.
- Others have not been able to replicate these results (possibly due to the difficulty in synchronizing entry).
- Repeated farming of the same area results in a progressive reduction of drops.
These hypotheses persist despite the fact that testing would take hundreds of players farming under controlled conditions for dozens of hours each. This does not make them untrue, just unconfirmable.
- The drop rate is affected by external factors:
- Too many players farming the same area.
- Time of day (e.g. peaking or hitting minimums every three hours)
- The drop rate decreases if your character or account does a lot of farming.
- This clearly affects drop rates for Treasure.
- This cannot be confirmed for foe drops (see above).
Using these suggestions can mitigate the effect of the anti-farm code:
- Avoid killing several foes at once; try to limit the usage of AoE and mass DoT skills. Make a reasonable pause between kills.
- When possible, try to add leechers to a party. With proper planning of a farming route, you will get more drops while they will get points/bounties from resurrection shrines. There is controversy as to the effects (if any) of leechers on drop rates.
- After entering the area, avoid rushing to a foe with a valuable drop like a boss. Kill some unimportant foes to be sure that the normal drop rate is restored.